Work has continued on methods and apparatus for the study of kidney and toad bladder epithelial cells grown as sheets on porous membranes. A device for sterile measurements of potential difference and short circuit current has been made. It is useful for routine monitoring of cell preparations during growth. This design is possible because we grow the cells on membranes which are cemented to polycarbonate rings, which provide electrical and chemical isolation between the solutions contacting each side of the membrane. A tonometer using silicone rubber tubing has been built for the study of the CO2 to HCO3-pk in physiological salt solutions with and without 6% albumin. The pK of the albumin solution was measured to be 0.02 lower. This could result from the binding of 5% of the HCO3-to the albumin as occurs for Cl.